In recent years, a GCR (ghost canceller reference) signal has been inserted in the vertical blanking period as the reference signal for de-ghosting in response to demands for high quality pictures in TV broadcasting. Transversal equalizers have been adopted by TV sets for cancelling ghost images through waveform equalization using the GCR signal.
Shown in FIG. 1 is a circuit diagram showing the transversal equalizer which is for real-time waveform equalization. The circuit shown in FIG. 1 is referred to in, for example, Shri Goyal et al., "Performance Evaluations of Selected Automatic Deghosting Systems for Television", "IEEE Transactions on Consumer Electronics", Vol. CE-26, pp. 100-120 (February 1980) and is called a 6-taps transversal equalizer.
An input signal is sampled every T seconds and this input sample value Xi is applied to the input terminal 1. This input sample value Xi is applied to the multipliers M1 through M6, respectively and multiplied by coefficients (hereinafter referred to as tap coefficients) C1 through C6.
Outputs of the multipliers M1 through M6 are applied to the adders A1 through A6, respectively. Outputs from the adders A1 through A6 are delayed by the delay units D1 through D6, respectively and applied to the output terminal 2 and the adders A1 through A5 in the next filter stage.
Further, the delay units D1 through D6 are driven by the clock CK with a period of T seconds, supplied through the input terminal 3, and output signals by delaying them by T seconds. At the output terminal 2, an output based on a tap coefficient appears and it is possible to equalize the transmission lines by setting the tap coefficient.
Further, signals from the preceding stage are input to the cascade input terminal 4. This signal is added to the output from the multiplier M6 in the adder A6.
Now assuming that an input sample value Xi is .delta. function {.delta.i}, its impulse response {.alpha.i} is .alpha.1=C1, .alpha.2=C2, . . . , .alpha.6=C6. As the length of a train of impulse responses is the same as the number of taps, a longer train of impulse responses is obtainable by increasing the number of taps. That is, a filtering time can be extended by increasing the number of taps.
The range of delay times in which ghosting can be cancelled by a GCR signal is 44.7 .mu.s of the GCR signal width. See, for example, Susumu Takayama et al., "Deghosting Reference Signal System", "1989 National Convention Record of the Institute of Television Engineers of Japan", pp. 239-240.
As described above, the length of the filtering time is decided by the number of taps and the number of taps required to obtain the range of 44.7 .mu.s delay time is expressed by 44.7 .mu.s/T (clock period). Normally, the clock frequency in the TV signal digital processing is set at 4 times the chromatic sub-carrier frequency (14.31818 MHz) and the clock period T is 69.84 .mu.s. That is, in the de-ghosting using GCR signal, as many as 640 taps are needed.
Transversal equalizers have normally been made in integrated circuits (IC) and with the advanced degree of integration, 64 taps have been integrated in a single chip like TF-IC adopted in Toshiba's ghost clean TV tuner (TT-GC9).
Shown in FIG. 2 is a block diagram showing a conventional transversal equalizer which has adopted such transversal equalizers. See, the example, "Ghost Image Reducing Tuner and its Operation", Japanese magazine "Chroma", pp. 48-51, (December 1989).
A video signal subjected to disturbance by a ghost image is input to the input terminal 5. In this input video signal a GCR signal has been inserted.
The input video signal is input to the input terminal 6 of the transversal equalizer. The cascade input terminal 7 of the transversal filter 11 is connected to the reference potential point. Further, the input terminals 6 and 7 correspond to the input terminals 1 and 4 shown in FIG. 1.
To respective taps, not shown, of 64 multipliers of the transversal filter 11, tap coefficients C-29 and CO are applied, respectively. Further, subscripts of the tap coefficients indicate which delay time of clock period T they correspond to.
The coefficient of the main tap corresponding to the rise of the GCR signal is CO. CO is set for 1 and other tap coefficients for 0 at the time of initialization. Therefore, in the state of initialization, the transversal filter 11 directly outputs the video signal input to the input terminal 6 to the output terminal 8.
This transversal filter 11 is of the non-recursive type and constructed to the range of delay times from -2 .mu.s (pre-ghost) to 2.4 .mu.s (delayed-ghost) by the tap weight factors C-29 through C34. That is, cancellation of waveform distortion (waveform equalization) and cancellation of ghost in short delay times (nearby-ghost) are performed by the transversal filter 11.
The output from the transversal filter 11 is applied to the output terminal 10 through the subtractor 9 and at the same time, to the delay unit 21 from the output terminal 10.
The output of the delay unit 21 is applied to the input terminal 6 of each of the transversal filters 12 through 20 in the same construction as the transversal filter 11. The number of taps of the transversal filters 12 through 20 is 64 and tap coefficients C35 through C610 are applied to the transversal filters 12 through 20. That is, the transversal filters 12 through 20 correspond to delayed-ghosts for delay times 2.4 .mu.s through 42.6 .mu.s.
Outputs of the transversal equalizers 20 and 12 are applied through each cascade input terminal 7 of the transversal filters 19 through 12 in the next stage from respective output terminals 8, to the subtractor 9, thus forming the recursive filter.
The coefficients C35 through C610 are set at 0 in the initial state and when tap coefficients are corrected thereafter, a ghost cancelling signal is output from the transversal equalizer 12. The subtractor 9 output a deghosted video signal to the output terminal 10 after subtracting the ghost cancelling signal from the output of the transversal filter 11.
Tap coefficients C-29 through C610 are obtained through operation of the GCR signal extracted from input/output video signals and reference signal and are successively corrected at specified time intervals. That is, the GCR signal contained in the video signal from the input terminal 5 and the GCR signal contained in the video signal from the output terminal 10 are extracted. The GCR signal contained in the output video signal is compared with a reference signal to get an error signal and further, correlated operation of this error signal with the GCR signal contained in the input video signal is carried out and tap coefficients are corrected to minimize the error signal.
Further, as described above, the same ICs are used for the transversal filters 11 through 20 from the viewpoint of cost.
As described above, waveform equalization and nearby-ghost image removal are performed by the non-recursive transversal filter 11 and long distant ghost and secondary ghost (see the above-mentioned magazine, "Chroma") produced by this transversal filter 11 are removed by the recursive transversal filters 12 through 20.
Series connection of the non-recursive and recursive transversal equalizers is the construction best suited for removal of ghost image as shown in the above-mentioned "Chroma".
Generally, as to pre-ghost images, if pre-ghost images of delay time less than 2 .mu.s are removed, there will be no problem in practical use. On the other hand, the delay time of delayed-ghost image may sometimes become more than 40 .mu.s.
Since the range of ghost image removable delay time is 44.7 .mu.s as described above, the corresponding range of pre-ghost image was set at -2 .mu.s in the example shown in FIG. 2. That is, tap coefficients of the transversal filter 11 are from C-29, through C0 to C34.
The circuit is of such a construction that output of the transversal filter 11 and the transversal equalizer 12 are subtracted in the subtractor 9 to remove ghost image components and outputs of the transversal filters 11 and 12 are delayed by T on time base and the delay unit 21 operates at the delay time 34T.
It is, however, considered that with the advancement of integration of digital IC, the number of taps that can be integrated in a single chip will further increase in the future. If so, the number of taps for delayed-ghost images of the transversal filter 11 will be increased.
In this case, therefore, it becomes also necessary to extend the delay time (the number of delay taps) of the delay unit 21 corresponding to the increase in the number of taps in order to delay the time base of outputs of the transversal filters 11 and 12 by T. This will increase the size of the circuit and its cost.
In the conventional transversal equalizer described above there was the problem that it becomes necessary to increase the delay time of the delay unit and the circuit size becomes large, with a corresponding increase in the number of taps in a single chip.